In semiconductor chip fabrication plants the wafers are typically subjected to a preliminary test to check for particles or other defects on their surfaces. If a particle or other defect is detected, the wafer may be cleaned or discarded, as appropriate. At present, the surfaces of the wafers are typically inspected with an optical system. Such optical systems are capable of detecting particles having widths of approximately 500 .ANG. or greater, but they are unable to detect smaller particles.
This limitation has not presented a problem until now, because particles smaller than about 500 .ANG. did not interfere with the fabrication processes. The line widths on microchips are becoming much smaller, however. Line widths of 0.35 .mu.m are now common, and they are moving towards 0.25 .mu.m. Eventually, they will reach 0.1 .mu.m (1,000 .ANG. units). At this scale, a particle having a width of, say, 300 .ANG. would cover about one-third of a line width and could easily result in a defective chip.
In a device according to this invention, particles having dimensions far less than 500 .ANG. can be detected on a semiconductor wafer prior to the commencement of processing.